It has long been known that certain complexes of aromatic di-amines, for example, alkali metal salt complexes of 4,4′-methylenedianiline (MDA), can be used to cure amine-curable polymers or prepolymers without premature reaction between the curable polymer or prepolymer and the aforementioned diamines. These amine complex curatives have found use in so called one-pack polyurethanes, i.e., urethane compositions containing isocyanate terminated prepolymers, formed by reacting a polyol and poly-isocyanate, and curative which are designed to not react until activated, typically by heating.
Commercially available curing compositions comprising coordination complexes of aromatic diamines and metal salts are formulated as dispersions in high boiling liquids, typically liquids widely known as plasticizers, such as phthalate diesters.
U.S. Pat. No. 3,876,604 discloses a dispersion of a coordination complex of MDA and an alkali metal salt in an inert liquid vehicle. The MDA complex is formed by reacting an aqueous solution or brine containing a sodium or lithium chloride, bromide, iodide or nitrite salt with a methanolic solution of MDA. The MDA coordination complex has a 3:1 mole ratio of MDA to alkali metal salt and is isolated as a crystalline precipitate e.g., by filtration, decantation, centrifuging or other suitable operation, with a particle size of ≧150 microns.
In order to be effective as a curing agent, e.g., a curing agent for a polyurethane prepolymer, the particle size of the complex needs to be less than 60 microns. The particle size of the MDA complex produced by the process of U.S. Pat. No. 3,876,604 is too large to be effective, i.e., ≧150, and so a grinding or milling step is required to produce effective material.
It was found however that relatively intensive mixing is required to adequately disperse the finely divided curing agent when added alone to a polymer or prepolymer, e.g., a polyurethane prepolymer. This is not just inconvenient but may result in some premature curing because of the heat generated from the mixing operation. A major advantage of using a dispersion of the MDA complex in an inert liquid vehicle is that the curing agent can be uniformly dispersed throughout the prepolymer or polymer without expending excess energy or generating the heat that would cause premature cure.
The inert liquid vehicle must be inert to both the complex and polymer or prepolymer being cured, be readily miscible with the prepolymer or polymer which is to be cured so that the liquid and polymer may be readily comingled, and in general must possess sufficiently low volatility so as not to vaporize from the cured polymer after curing or during end-use applications. One exception to the low volatility requirement is in applications where the loss of a solvent-like material could take place without difficulty such as in surface coatings and if the liquid vehicle is sufficiently low boiling, such as methylene chloride, it can serve as an expanding agent during the curing operation to yield cellular products.
The importance of the mutual compatibility of the liquid phase of the curing agent dispersion and the polymer or prepolymer being cured is illustrated by the following example. Nujol oil, a saturated light hydrocarbon petroleum oil, readily forms a stable dispersion with the complex; however, the resultant mixture is incompatible with polyether polyurethanes, among other polymers and prepolymers, and thus Nujol dispersions are considered ineffective for curing such prepolymers.
Examples of suitable liquid carriers found in the art include esters of phthalic acid, isophthalic acid, trimellitic acid aliphatic diacids such as adipic, azeleic and sebacic acids, aromatic and naphthenic hydrocarbon processing oils, halogenated biphenyls and liquid aromatic sulfonamides. Specific examples include di(2-ethylhexyl)phthalate, tetraethylene glycol bis(2-ethylhexanoate), and an aromatic process oil comprising 18% polar compounds, 76% aromatics and 6% saturated petroleum derivatives. Paraffinic hydrocarbon oils have limited compatibility with most of the well-known amine-curable polymers and therefore are considered to be of value only on rare occasion.
U.S. Pat. No. 3,899,438 discloses that a dispersion of small particles of a MDA/alkali metal salt coordination complex can be formed in a suitable inert liquid carrier in a one step process, i.e., without a separate milling step, by running the reaction in a high shear mixer, such as a ball mill or high speed disperser run at 7,000 rpm. Suitable inert carrier liquids include di(2-ethylhexyl)phthalate and tetraethylene glycol bis(2-ethylhexanoate). U.S. Pat. Nos. 4,282,344 and 4,075,150 disclose a process similar to that of U.S. Pat. No. 3,899,438 wherein excess MDA is removed by reaction with added isocyanate (U.S. Pat. No. 4,282,344) or an aromatic mono-carbodiimide (U.S. Pat. No. 4,075,150). A similar process is also disclosed in U.S. application Ser. No. 12/754,944, now U.S. Pat. No. 8,586,682.
Commercially available curing compositions comprising coordination complexes of aromatic diamines and metal salts are formulated as dispersions in high boiling liquids, typically liquids widely known as plasticizers, i.e., aromatic diesters such as dioctyl phthalate, and alkyl diesters such as dioctyl adipate. When the curing composition is added to a prepolymer and the mixture is heated and cured, the plasticizer remains in the product. Plasticizers have negative effects on polyurethane polymers such as reduced dynamic and mechanical properties and environmental, health and safety issues due to the partial volatilization of plasticizer (smoke formation) during cure have been encountered. There is therefore a need for a readily prepared and highly effective curative composition for amine curable polymers that avoids the use of the current plasticizer carrier liquids and the problems caused by them.